RC Time Constant Calculator
Calculate time constant, charge time and cutoff frequency for an RC circuit.
Results
The RC time constant determines how fast a capacitor charges or discharges through a resistor. After one time constant (τ = RC), the capacitor reaches 63.2% of its final voltage. After 5τ it is 99.3% charged. This same RC combination also sets the cutoff frequency for low-pass and high-pass filters - a fundamental building block of analog electronics.
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Frequently asked questions
τ = R × C = 1000 × 0.0001 = 0.1 seconds (100 ms). The capacitor reaches 63.2% charge in 100 ms and 99.3% in 500 ms.
Theoretically never, but after 5 time constants the capacitor is 99.3% charged. For a 100 ms time constant, that is 500 ms. Engineers consider this 'fully charged' for practical purposes.
fc = 1/(2πRC). With R = 1 kΩ and C = 100 µF, fc = 1/(2π × 0.1) = 1.59 Hz. Signals above this frequency are attenuated by a low-pass filter.
The time constant doubles, which halves the cutoff frequency. A 2 kΩ / 100 µF circuit has τ = 200 ms and fc = 0.796 Hz, compared to 100 ms and 1.59 Hz with 1 kΩ.
Yes, RC timing is used in 555 timer circuits and debouncing. A 10 kΩ / 10 µF combination gives τ = 100 ms, useful for button debouncing. The 555 timer charges to 2/3 Vcc in about 1.1RC.